The present invention relates to a method and apparatus for converting natural gas to liquid hydrocarbons for storage and transport. More specifically, the invention relates to such a method and apparatus which are especially suitable for fixed offshore hydrocarbon production platforms.
A large number of fixed offshore hydrocarbon production platforms are currently in operation, all of which produce various quantities of natural gas. At many such sites natural gas is produced in relatively small quantities as a byproduct to the production of oil. When the quantities of this “associated gas” are sufficiently large and the platform is sufficiently close to a gas transportation infrastructure, the gas can be transported to an off-site processing facility.
In the absence of a gas transportation infrastructure, the associated gas is typically disposed of using a variety of methods. One of these methods involves re-injecting the gas into the hydrocarbon formation via a gas injection module located on the platform. Another method entails burning, or flaring, the gas. However, these methods of disposing of the associated gas are wasteful and, in the case of flaring, environmentally unfriendly.
Also, a number of small to medium size gas fields exist which are currently considered “stranded”. Stranded fields are those which are located too far from an existing gas transportation infrastructure to be economically feasible to produce.
As is readily apparent, unprocessed associated gas and stranded gas represent two considerable but untapped sources of hydrocarbons. These sources are not utilized because the transportation of the gas from remote fixed offshore platforms is presently uneconomical. However, the transportation of the gas would be more economically viable if it could first be converted into liquid hydrocarbons, because then it could be transported using the existing oil transportation infrastructure.
Gas-to-liquids (“GTL”) technology is commonly employed to convert natural gas to liquid hydrocarbons. The most common GTL process is a two stage process in which the natural gas is first converted into a synthetic gas, or “syngas”, and the syngas is then converted into liquid hydrocarbons using the Fisher-Tropsch process. The conversion of natural gas to syngas may be achieved by steam reforming, partial oxidation, or a combination of both. Steam reforming, which is an endothermic process that is performed in a catalytic reactor, typically produces syngas comprising a 3:1 ratio of hydrogen to carbon monoxide. In the Fisher-Tropsch process, which is also carried out in a catalytic reactor, the small hydrocarbon molecules are linked to form longer chain hydrocarbons that are primarily liquid at ambient temperature.
Although GTL technology has been in the public domain since the 1930's, current GTL technology is generally not considered economical at small scales, especially scales suitable for use on fixed offshore hydrocarbon production platforms. To date, reducing a GTL facility down to the size required to replace an existing gas processing facility and at the same time fit into the confined footprint available on a fixed offshore platform has not been feasible. Previously, any GTL facility which could fit into this limited space typically would not have sufficient processing capacity, that is, it could process only unfeasibly small quantities of natural gas into liquid hydrocarbons.